>>10152979I can't accuse you of doing this because I don't know what you actually wrote, but I've graded abstract algebra exams and every time I get at least a few students who complain that I gave them minimal marks for an argument with a fundamental flaw in it.
If you write a proof that has an error, and the error breaks the proof in a way that cannot be fixed, then it doesn't matter if the other 95% is correct because that one wrong step means your entire solution makes no progress at all on the question you were asked.
Part marks are supposed to indicate that you made partial progress towards a solution. If you made no progress, you cannot reasonably get any marks.